It is proposed to study the kinetics and biochemistry of the recovery from radiation-induced cycle delay as a function of stage in the cell cycle in synchronized Chinese hamster cells. Answers are being sought to the following questions: 1) What is the nature of the two component repair process of radiation-induced division delay? At what level of damage does the rapid, initial repair process become operant and what is the nature of its dose-effect characteristics? Do both components have the same activation energy? These answers can be derived by varying the size of the first dose in two dose fractionation schemes and by reducing the temperature during the fractionation level. 2) Does the recovery from radiation-induced S delay occur with the same kinetics as division delay? 3) The activation energy of the repair process for division delay in asynchronous populations is 11,020 cal/mole, is it the same for radiation-induced S delay? Does the activation energy of the repair process vary with cell age? This can be readily determined at different stages of the cell cycle by reducing the post-irradiation temperature. 4) What is the effect of inhibiting RNA or protein synthesis on these repair processes? Answers can be attained by the application of lucanthone or cycloheximide between two dose- fractions respectively. 5) What are the kinetics and the biochemical nature of the recovery from hyperthermia induced cycle delay? 6) How does the damage from radiation and the damage from hyperthermia interact at the level of impaired cycle progression? 7) What is the effect of gene dosage on the recovery from radiation-induced cycle delay as determined by applying the above experimental procedures to our strain of tetraploid Chinese hamster cells? This information will allow a greater understanding of the role of cell cycle progression, parasynchronization , and repopulation between fractionated doses of irradiation and hence be able to predict how these parameters might be more fully utilized to enhance the effectiveness of radiation in human cancer therapy.